This adds a //sql/mojo library which can be linked into preexisting
sqlite3 code which adds a new VFS which transparently proxies filesystem
usage to the mojo:filesystem application.

We create a new sql_apptests.mojo target, which currently has all the
sql connection_unittests.cc (minus 2 hard ones), all statement and
transaction unit tests and refactor the sql testing stuff so that we
have two implementations of an SQLTestBase class: one that uses files
raw and one that proxies to the filesystem process.

Notably, this patch does not implement file locking, which will have to
be implemented before we can safely use this, but will be a large enough
patch in and of itself that I'm punting on it for this patch.

BUG=493311

Review URL: https://codereview.chromium.org/1176653002

Cr-Commit-Position: refs/heads/master@{#335415}